1. Field of the Invention
The present invention relates to an apparatus and a method for drying semiconductor fragment material.
2. The Prior Art
High-purity semiconductor material is required for the production of solar cells or electronic components, such as memory elements or microprocessors. The semiconductor material is, for example, silicon, indium phosphide, germanium, gallium arsenide or gallium phosphide.
The deliberately introduced dopants are the only "impurities" which a material of this type should have in the most favorable case. It is therefore desirable to keep the concentrations of damaging impurities as small as possible.
It is frequently observed that semiconductor material that has already been produced with high purity is contaminated anew during further processing to provide the desired products. Thus, costly treatment/cleaning steps with subsequent drying operation are repeatedly necessary in order to regain the original purity. Impurity metal atoms which are incorporated into the crystal lattice of the semiconductor material disturb the charge distribution and can reduce the function of a subsequent component or can lead to the failure thereof. Consequently, it is necessary to avoid contamination of the semiconductor material by metallic impurities. However, other impurities or particles on/in the surface of the semiconductor fragment material can also have a lasting adverse effect on the subsequent melting process and lead to reject material.
This applies to silicon, which is the most frequently used semiconductor material in the electronics industry.
High-purity silicon is obtained by chemical reaction of the raw silicon into a liquid silicon compound (for example trichlorosilane). This can be worked up to a form of ultra-high purity with the aid of distillation processes. In a subsequent chemical deposition process, this high-purity silicon compound is then converted into high-purity silicon. It is obtained as an intermediate product in the process as polycrystalline silicon in the form of rods.
The same applies analogously to the other semiconductor materials. They, too, are predominantly produced firstly as a polycrystalline intermediate product.
Most of this polycrystalline semiconductor material is used for the production of crucible-pulled single crystals, or for producing tapes and films. It can also be used for the production of polycrystalline solar cell base material.
Since these products are produced from high-purity, molten semiconductor material, it is necessary to melt solid semiconductor material in crucibles.
To make this operation as effective as possible, it is necessary to produce large-volume solid semiconductor pieces of defined fragment size distribution. There are precisely specified requirements made of the surface purity for technical process reasons. No impurities are allowed to pass into the crucible with the semiconductor fragment material. The surface of the semiconductor fragments must be dry and free from dust and acid. Otherwise--particularly in the case of single crystal growth--impurity particles lead to dislocations and lattice defects and make it impossible to effect crystal growth successfully.
In order to produce high-purity semiconductor fragment material, the polycrystalline semiconductor material (such as the above-mentioned polycrystalline silicon rods) or indeed monocrystalline semiconductor recycling material is comminuted before being melted. This is usually associated with superficial contamination of the semiconductor fragment material. This is because the comminution is predominantly carried out using mechanical breaking tools, such as metallic or ceramic jaw or roll-type crushers, hammers or chisels. As a result of the comminution operation, impurity atoms (iron, chromium, nickel, copper, etc.) are worked into the surface of the semiconductor material or adhere to the surface. However, even in the alternative breaking methods, such as water jet breaking, shock wave comminution, etc., it is not entirely possible to preclude such contamination. Examples of this contamination are with impurity atoms or the possibility of damaging dust and/or particles from reaching the fragment surface.
In particular, contamination by metal atoms is to be regarded as critical since these can alter the electrical properties of the semiconductor material in a damaging manner. Dust and/or particles on the surface can have a lasting adverse effect on the subsequent pulling process (dislocations, etc.).
In order to be able to use mechanically processed semiconductor material as starting material for the further production process, the following is necessary. First, it is necessary to reduce the concentration of metal ions and particles which have made their way onto or into the surface of the mechanically processed semiconductor material as a result of the processing operation and handling.
Thus, before being melted, the semiconductor fragments must be subjected to a chemical surface treatment with subsequent cleaning and drying in order to achieve the specified purity values for the surface.
For this purpose, the surface of the mechanically processed semiconductor material is etched using diverse acids, such as a mixture of nitric acid and hydrofluoric acid. This process is widely used. Afterwards the semiconductor fragment material, for example, polycrystalline silicon fragments, is usually rinsed with ultrapure water and dried. Since no impurities are allowed to pass into the crucible with the semiconductor material, the surface/surface structure of the semiconductor fragment material must be absolutely dry and free from dust, specks and acid.
Semiconductor material is very brittle. Therefore, the breaking operation yields a sharp-edged, fissured semiconductor fragment material having a multiplicity of fine hairline cracks. These cracks will have propagated as far as the cm range under the surface. In particular, residual moisture (water and acid residues) forms in these cracks on account of the capillary effect. This residual moisture can subsequently lead to contamination (specks), to reject material, or even to cauterization. In order to fulfill the high quality requirements, which are continually being made more stringent, satisfactory drying is required. That is to say acid-free and speck-free semiconductor fragment material is absolutely necessary.
Conventional convection drying is by sending a stream of ultrapure air over and/or through the material being dried. This does not afford the success hoped for in an appropriate period of time (less than one hour), which can be discerned inter alia from the coloration of litmus paper, unless complicated, bulky and thus costly equipment is set up or the material is stored unpackaged "in the open" for a relatively long period of time. In this case the risk of intensified dust contamination is very high. A further disadvantage of convection drying is that moisture remains in the extremely fine hairline cracks and thus the risk of subsequent specking/dust contamination is increased. This leads to a quality deterioration and possibly even to rejects.
In the case of radiation drying, the upper layer is primarily heated, with the result that areas on the "shadow side" of the semiconductor fragment material are not heated sufficiently. Also, in the case of beds, layers deeper down are not sufficiently included. Furthermore, the removal of acid from the hairline cracks is not entirely satisfactory. This likewise leads to specking, that is to reject material.
If the radiation intensity is increased, then the surface temperature can be increased to above 100.degree. C. This will cause metal ions that have not been cleaned away to diffuse, as the temperature increases, into the surface of the semiconductor fragment material. This will contaminate the pure semiconductor material in a sustained manner. This leads to a quality deterioration and possibly even to rejects.
The same applies analogously to drying with the use of microwaves. Here, too, diffusion of damaging metal ions into the semiconductor material will occur. Reject material is to be expected on account of the heating of the material.
Drum drying is also not practical. This is because drum abrasion occurs as a result of the movement of the fragment material between semiconductor fragment material and process drum, on the one hand. Also, between the semiconductor fragments themselves, on the other hand, there is sustained drum abrasion and/or semiconductor fine fragments/dust will occur. As a result of this the subsequent pulling process is greatly impaired (high dislocation rate) and likewise leads to reject material.